The present invention is directed generally to devices for small charge blasting and specifically to devices for small charge blasting having a retractable gas generator.
In mining and civil excavation work, small charge blasting or controlled fracture techniques are being introduced as alternatives to conventional drill-and-blast, mechanical breakers, chemical expansion agents and, in some cases, hand methods. xe2x80x9cSmall charge blastingxe2x80x9d as used herein includes any excavation method where relatively small amounts of an energetic substance (typically a few kilograms or less) are consumed for each hole in a rock breaking sequence or where a pressurized fluid is sealed in the bottom of a drill hole by a stemming member to initiate and propagate a fracture in the material to be broken. xe2x80x9cSealingxe2x80x9d refers to the partial or total blockage of the hole to impede escape of the fluid from the hole. Examples of small charge blasting devices and methods are described in U.S. Pat. Nos. 5,765,923; 5,308,149; and 5,098,163.
In many small charge blasting methods, a machine drills a hole into the rock to be broken and then inserts an elongated stemming member, which can be a stemming bar, a gas injector barrel, or other pressurizing device, into the hole. A pressurized working fluid, such as a liquid, gas, or another type of pressurizable fluid, is released rapidly into a portion of the hole, usually the bottom portion. The pressurized fluid is typically generated by combustion of a propellant or explosive source, by an electrical discharge into a conductive fluid, or by mechanical compression of the working fluid. The stemming member seals and stems the pressurized working fluid in the hole bottom and thereby causes fracturing of the rock. Small charge blasting can be highly mechanized and automated so that it can be carried out more or less continuously to increase productivity, can permit excavation machinery to remain near the face due to reduced fly rock discharge, and can have a seismic signature that is commonly relatively small because of the small amount of blasting agent used in the blasting sequence.
In designing a small charge blasting apparatus, there are a number of considerations. It would be desirable to use an inexpensive cartridge for providing the working fluid. By way of example, an all plastic cartridge housing could offer significant cost savings. It would also be desirable to have the drill and stemming member on a common boom to simplify the sequential alignment of the drill that forms the hole and the stemming member that is subsequently inserted into the hole. The boom is typically attached to undercarrier or other mounting mechanism. It would also be desirable to maintain an accurate and stable boom alignment relative to a point of reference on the excavation face for both the drilling process and the subsequent process of stemming member insertion. This is often difficult with an indexer mechanism (such as that shown in FIG. 8 of U.S. Pat. No. 5,098,163) which is front heavy and makes it difficult to maintain accurate alignment. Thus, the weight and moment arm of the boom should be substantially minimized not only to maintain an accurate and stable boom alignment but also to permit the use of lighter weight hydraulic systems and under carriers. Finally, the drill and stemming member should be positioned on the boom to permit the spent cartridge in the stemming member to be replaced with a new cartridge during drilling of the hole. If the drill and stemming member are located in close proximity to one another, mud and rock particles from drilling can enter the breech of the stemming member when the spent cartridge is in the process of being replaced with a new cartridge. This can lead to misfires or plugging or jamming of the breech and/or loading mechanism. Additionally, the mud and rock particles can plug the barrel of the stemming member and thereby cause damage to the stemming member when the pressurized working fluid expands down the barrel.
These and other objectives are realized by the various embodiments of the present invention.
In a first embodiment of the present invention, a small charge blasting device is provided that includes:
(a) a stemming member (e.g., a stemming bar, a gas-generator barrel, or other device for sealing the hole to pressurizing the working fluid in the hole) for insertion into the hole and
(b) a slide assembly comprised of:
(i) a guide track and proximal and distal ends;
(ii) a drive mechanism (e.g., a belt drive, a chain drive, a worm gear drive, a pusher rod drive, a cable drive, a hydraulic or gas powered extending cylinder, or another suitable drive device) located adjacent to the guide track for moving the stemming member linearly back and forth along the track between the proximal and distal ends of the slide assembly (i.e., between its stowed and deployed (or working) positions); and
(iii) a switching mechanism (e.g., a rotating or translating cradle mechanism, a lifting clamp mechanism, or a tracked switching mechanism) located adjacent to the drive mechanism at the proximal end of the slide assembly for transferring the stemming member from a stowed position at the proximal end of the slide assembly onto the guide track for transport to a deployed position at a distal end of the slide assembly. The ability of the stemming member to be retracted to the proximal end of the slide assembly for stowing lowers the weight and moment arm of the distal end of the boom supporting the slide assembly during positioning of the slide assembly by the boom and therefore permits the use of lighter weight (and less expensive) positioning hydraulics and simplifies alignment of the slide assembly with the hole. The lighter slide assembly weight also enables the slide assembly alignment to be maintained more accurately and with more stability since the large dynamic forces associated with a front-heavy indexer mechanism are substantially eliminated.
The small charge blasting device can include another tool for performing another type of unit operation such as a drill for forming a hole in a material to be broken. When stowed, the second tool is typically located on the proximal end of the slide assembly adjacent to the stowed stemming member and on an opposite side of the guide track from the stowed stemming member. The second tool and stemming member are alternatively and sequentially advanced by the drive mechanism along the guide track after being switched onto the guide track from their respective stowed positions to the excavation face to perform their separate functions. When the second tool is a drill that is located at the distal end of the slide assembly when it performs its drilling function, the stemming member will be in its stowed position at the proximal end of the slide assembly. Accordingly, the spent cartridge (from a previous shot) may be removed and a new cartridge inserted into or onto the stemming member during the drilling process without drilling water, mud and other debris entering the barrel, breech, or cartridge loading mechanism of the stemming member. The second tool and stemming member are thus located on a common boom and can be readily, simply and sequentially aligned with a desired location on the excavation face.
During operation, the drill (or other type of tool) and stemming member may be alternately and sequentially engaged with or disengaged from the drive mechanism and/or guide track by the switching mechanism. Because the dynamic forces are relatively low, the drill typically remains engaged with the drive mechanism during the drilling process. In contrast, because of the relatively large dynamic forces, the stemming member is typically disengaged from the drive mechanism during formation of the controlled fracture in the material. The significant recoil forces exerted on the stemming member resulting from fracture initiation and propagation can damage the drive mechanism.
The slide assembly can include one or more features to dampen recoil of the stemming member in response to the generation and/or release of the pressurized working fluid into the hole. For example, the guide track can include a clamping device (e.g., which is operated hydraulically, electrically, magnetically, or mechanically) for clamping the stemming member in the guide track to dissipate recoil energy of the stemming member after release of the working fluid into the hole. By way of illustration, the clamping device can be configured such that it clamps down on the plate on which the stemming member is rigidly attached. The guide track can also include a braking material having a static coefficient of friction of at least about 0.2 and more preferably ranging from about 0.5 to about 1.0 and a sliding coefficient of friction of at least about 0.2, more preferably ranging from about 0.2 to about 1.0, and even more preferably from about 0.2 to about 0.5 to dissipate the recoil energy of the stemming member as the stemming member recoils and moves along the guide track. In this manner, the guide track acts as a long brake pad to dissipate the recoil energy. The static force with which the stemming member would be held in the drill hole in the firing position would be dictated by the contact area between the plate attached to the stemming member and the guide track and the static coefficient of friction. The static force typically ranges from about 3,000 to about 25,000 lbs. Likewise, the retarding or breaking force resisting the recoil after firing would be dictated by the contact area between the plate and the guide track and by the sliding coefficient of friction. The retarding or braking force typically ranges from about 1,000 to about 25,000 lbs. If the recoil motion of the stemming member is terminated along the guide track, the clamping device would be disengaged and the drive mechanism re-engaged to move the stemming member to the proximal end of the guide track. Finally, either the slide assembly or the plate on which the stemming member is rigidly attached can be engaged with a shock absorbing device positioned behind the stemming member to dissipate either the full recoil energy should the clamping device fail or not be available; or any residual recoil energy not dissipated by the clamping device.
In a second embodiment of the present invention, a small charge blasting method is provided that broadly includes the following steps:
(a) engaging the stemming member with at least one of the drive mechanism and the guide track;
(b) advancing the stemming member linearly along the guide track on the slide assembly (e.g., from the stowed position to the deployed position) to insert the stemming member into the hole in the material to be broken;
(c) when the stemming member is positioned in the hole, pressurizing the working fluid in the hole to fracture the material to be broken;
(d) thereafter retracting linearly the stemming member along the guide to the stowed stemming member position; and
(e) disengaging the stemming member from the at least one of the drive mechanism and guide track.
When the device includes a drill or another type of tool mounted on the slide assembly with the stemming member, the method further includes the following steps either before the engaging step (a) or after the disengaging step (e):
(f) engaging the drill with at least one of the drive mechanism and guide track;
(g) advancing the drill (or other tool) linearly along the guide track (e.g., from the stowed position to the deployed position) to form the hole;
(h) retracting the drill linearly along the guide track, after hole formation, to the stowed position; and
(i) disengaging the drill from the at least one of the drive mechanism and guide track. These steps are repeated, blasting sequence by blasting sequence, to break the material.
In a specific application of the method broadly set forth above, the following specific steps are performed:
(a) switching the drill onto the guide track from the stowed drill position;
(b) engaging the drill with the drive mechanism;
(c) advancing the drill (or other tool) linearly along the guide track to the deployed drill position;
(d) forming the hole;
(e) retracting the drill (or other tool) linearly along the guide track;
(f) disengaging the drill (or other tool) from the guide track and the drive mechanism;
(g) switching the drill (or other tool) to the stowed drill position;
(h) switching the stemming member from the stowed position onto the guide track from the stowed stemming member position;
(i) engaging the stemming member with the drive mechanism;
(j) advancing the stemming member linearly along the guide track to the deployed stemming member position (e.g.,to insert the stemming member into the hole):
(k) disengaging the drive mechanism from the stemming member;
(l) optionally clamping the stemming member to the slide assembly;
(m) when the stemming member is inserted into the hole, pressurizing the working fluid in the hole to fracture the material to be broken;
(n) unclamping the stemming member from the slide assembly;
(o) reengaging the stemming member with the drive mechanism;
(p) retracting the stemming member linearly along the guide track;
(q) disengaging the stemming member from the guide track and the drive mechanism; and
(r) switching the stemming member to the stowed stemming member position. During one or more of steps (a) to (g), the spent cartridge in the stemming member from a previous shot may be replaced with a new cartridge. These steps are repeated hole-by-hole to remove the desired material.
In a third embodiment of the present invention, the stemming member includes the following specific components:
(a) a breech for receiving a cartridge, the cartridge containing an energetic substance for generating a pressurized working fluid;
(b) a cartridge ejector mechanism (e.g., a manual or hydraulically actuated push or ejector rod, a pneumatic ejection system, a mechanism that grabs and pulls the cartridge forward, or any other extraction mechanism common in ordinance technology) that expels the (spent) cartridge through a front portion of the breech after generation of the pressurized working fluid; and
(c) a barrel to be received in a hole in a material to be broken. The barrel is in communication with the front portion of the breech to release the pressurized working fluid into the bottom of the hole. The barrel and breech are preferably both located in the hole.
The cartridge may be inserted into the breech in any desirable manner. For example, the cartridge may be inserted into the breech through the back end of the breech (the breech being a so-called mobile breech mechanism), and chambered in a combustion chamber. Alternately, the cartridge may be inserted through the front or side of the breech directly into the combustion chamber (the breech being a so-called permanent or immobile breech).
In one configuration of the stemming member in the third embodiment, the spent cartridge is ejected through the front or distal end of the device by a push rod otherwise called an ejector rod. The cartridge is preferably inserted into the breech by being pushed into place and thereafter ejected (after use) by being pushed out of the breech and out of the downhole end of the barrel. This configuration eliminates the need for the cartridge to be pulled by the ejector mechanism, which typically requires a cartridge extractor mechanism and extractor grooves in the cartridge. By pushing the cartridge for insertion and ejection, the rear of the cartridge can be formed such that there are no grooves, voids or spaces in the rear of the cartridge. In other words upon chambering the cartridge in the combustion chamber, the rear of the cartridge is in intimate contact with the face of the breech and the walls of the combustion chamber. The absence of voids or spaces removes the requirement for the base of the cartridge base to have enough mechanical strength to resist collapse of the base material into any voids or spaces during the combustion process. Thus the cartridge housing can be formed from relatively low-strength and inexpensive materials such as plastics (e.g., polyethylene, polypropylene, nylon, or co-polymer combinations of these). Alternately, the cartridge housing can be formed from a combustible material (e.g., typically comprised of nitrocellulose, cellulose, wood pulp and resins) so that the cartridge housing is entirely consumed in the combustion and there is no cartridge housing to eject. Alternately, the cartridge can be formed from the energetic substance alone and have no outer housing such as a cartridge formed by a foamed explosive or propellant combination.
In another configuration of the stemming member in the third embodiment, the combustion chamber is located at the distal end of the device so that when the device is inserted to the bottom of the drill hole, the front end of the cartridge is in close proximity to and substantially in communication with the bottom of the hole. The pressure developed in the bottom of the hole is substantially the same as the pressure developed in the combustion chamber.
The outside diameter of the distal (i.e., downhole) end of the stemming member can be slightly less than the diameter of the drill hole to form a sealing band. The pressurized fluid in the hole bottom is substantially impeded from leaking via the sealing gap between the sealing band portion of the barrel and the wall of the hole. Alternately and preferably, the outside diameter of the distal (i.e., downhole) end of the stemming member is significantly smaller than the hole diameter and the sealing band (which is located between the distal end of the stemming member and the hole opening) for a length along the barrel equal to at least about 50% of the diameter of the hole bottom. The sidewall of the hole adjacent to the distal end of the stemming member is pressurized to the same level as the hole bottom which promotes conditions for more efficient penetrating cone fracture formation and propagation.
To provide a relief volume for the controlled expansion of the working fluid and/or to permit the cartridge to be extracted, the downhole end of the bore in the barrel expands (e.g. is tapered) outwardly. In this manner, the pressurized fluid expands into the relief volume in the bore prior to release into the hole.